First, it may be stated that the containers, tanks, or electrochemical cells are filled with an electrolytic solution made up of, among other components, the metal to be deposited, and in which a plurality of anode/cathode pairs are submerged, in alternating positions, which, when supplied with electrical current, deposits the metal on the cathodes.
This means that when vertical anodes are used in said cells for the electrowinning of metals, said vertical anodes are constituted of a hanging structure, based on a horizontal current supply conductor bar and vertical distribution bars connected to the current supply bar, the distribution bars of which are defined by a copper or aluminum core, and a titanium outer layer or skin.
The coated or uncoated titanium anode plates are therefore electrically connected to the distribution bars, with the anodic electrolysis operation taking place on the surface of anode plates.
Conventional anodes present multiple combinations in terms of the number of vertical bars per horizontal current supply bar.
As indicated, the conventional vertical bars used in processes for electrowinning of metals are bimetallic bars with a copper or aluminum core and a titanium outer layer or skin. Copper or aluminum present the low electrical resistivity that is necessary for the effective transmission of large currents and the titanium protects the copper or aluminum against the chemical attack of the electrolyte, while at the same time allowing the connection of the coated or uncoated titanium anode plates to these vertical bars.
In this way, the anodes, and specifically the anode surfaces used in the processes for electrowinning of metals, in order to obtain optimum output and maximum capacity, operate close to the cathodes and have a large surface area in relation to the short anode-cathode distance, for example, a surface of 100×100 centimeters with a separation of 5 cm. This inevitably introduces the risk of electrical contact between the anode and cathode, or in other words, a short-circuit, in the case of any deformation or alteration of the flatness at any point on the cathode surface.
Cathodic surfaces are unstable by nature because their thickness changes quickly during the production process itself, and also because an increase in the thickness of the cathode at a single point on its surface reduces the anode-cathode distance, which reduces the electrical resistance, and applying Ohm's Law, increases ionic current at the point in question.
Increasing the current or ionic deposition increases the thickness of the metal deposited on said points, such that these events clearly present a positive feedback system, that, as we know, are intrinsically unstable processes, which in these cases end up creating anode-cathode contact, or a short-circuit.
Moreover, any alignment error or existing mechanical deformation will also cause direct anode-cathode contact, or a short-circuit.
Once direct electrical contact has been established between the anode and cathode, the potential electrochemical barriers between the electrolyte and the anode disappear, and the relatively high resistance of the electrolyte will also be eliminated. In these circumstances, the electrical current spikes to unacceptable values, damaging or destroying the coated or uncoated titanium anode plate, at the same time causing significant production losses.
Also, in the work process of the anodes in a cell for the electrowinning of metals, and specifically copper, oxygen bubbles with sulfuric acid are generated, a phenomenon known as “acid mist”. This “acid mist” creates a serious environmental contamination problem and can directly affect the health of plant operators, requiring the use of masks in cell rooms and the deterioration of the environment in the area in which the plant is located.
For example, in case of copper electrowinning, the electrolyte is mainly made up of a solution of sulfuric acid and copper sulfate. In its normal electrolysis process, the anode generates oxygen bubbles that are contaminated, holding sulfuric acid; a large part of these bubbles leave the electrolyte and form part of the surrounding atmosphere creating what is known as acid mist.
Moreover, a current limiter is a device that reacts to and cancels any current above a particular value, this value is characteristic of the specific device or model.
A very familiar example is the fuses in our homes; when there is a short-circuit or direct contact of the wires in the network, the fuse is blown and disconnects, leaving us in the dark. We must then reset or replace the fuse to provide lighting again. We use this example to explain the concept of resetting and to go into more detail in regard to the possibility of having the lights come back on automatically, after a period of time, if the physical short-circuit is no longer present, and with no external intervention. In this case, the fuse is an automatic reset fuse.
There are two ways to protect against short-circuits; one is canceling or forcing the current to zero, and the other is modulating the current to admissible lower values. Both cases are considered to be current limiters, but we will call the former digital on-off limiters, and the latter analog limiters.
We can also cite patent document WO 2015/079072, which describes an anode structure for the electrowinning of metals, which comprises a horizontal support bar and vertical bars, coated with plastic or epoxy, to which anode plates, called sub-meshes with an area of 25 to 225 cm2, are attached to which electricity is supplied by means of the respective wiring and/or printed circuits, which are protected by a series of insulating structures, and that are installed inside the bars coated with plastic or epoxy.